This invention relates to a piezoelectric coaxial cable and to a method of preparing the cable.
In general terms, a piezoelectric coaxial cable, comprises a central conductor, an intermediate piezoelectric layer surrounding the central conductor, and an outer conductor surrounding the piezoelectric layer. Such piezoelectric coaxial cables have been proposed for use as transducers since, when they are subjected to an applied pressure, for example caused by the impact of an object, or to acoustic pressure changes, a potential difference will be generated between the conductors by the piezoelectric material. Applications for such devices are numerous and include underwater towed-array hydrophones, intrusion detectors, and the like.
In recent years certain polymeric materials, for example poly(vinylidene fluoride) (PVF.sub.2) and vinylidene fluoride copolymers have been suggested for use as piezoelectric materials. In order to maximize the piezoelectric properties of a vinylidene fluoride polymer, it is necessary to orient the polymer by stretching it, preferably up to its "natural" draw ratio of about 4:1 or beyond, in order to convert at least a portion of the polymer from its initial alpha or form II crystalline phase into its beta or form I crystalline phase. Simultaneously with, or subsequent to, the stretching operation, it is necessary to polarize the polymer by applying a high electric field gradient across the polymer in a direction perpendicular to the direction of orientation in order to align the dipoles in the beta phase of the polymer. Electric field gradients of from 5 to 200 MV/m are typical for the polarizing operation, the maximum applied field gradient usually being determined by the dielectric breakdown strength of the polymer material.
In order to maximize the piezoelectric response of a piezoelectric coaxial cable the intermediate piezoelectric layer would need to be stretched axially and polarized radially between a central conductor and an outer conductor. While the outer electrode may be applied to the intermediate layer after stretching, or, if a corona polarizing method is employed, the cable may be passed through a corona discharge electrode and an outer conductor for the cable be subsequently provided, significant problems are encountered in the provision of an inner electrode for the cable. It is not possible to extrude the piezoelectric layer onto a conventional metal conductor, e.g., a copper conductor, in that it would then be impossible subsequently to stretch the intermediate layer in order to convert it into the beta-phase. This problem is particularly acute when attempting to make long lengths of piezoelectric coaxial cable.
One solution which has been proposed is to manufacture a piezoelectric coaxial cable by preparing a tape of the piezoelectric polymer, stretching it, polarizing it, and then wrapping it around the inner conductor. (See, for example, U.S. Pat. No. 3,798,474 to Cassand et al and U.K. Patent Application No. 2,042,256 to Quilliam.) However, this process is disadvantageous in that it requires numerous steps and may result in poor electrical contact between the piezoelectric polymer and the inner conductor.
U.S. Pat. No. 4,303,733 to Bulle discloses filaments which are essentially coaxial cables comprising at least three layers, at least two of which electrically conductive with at least one electrical insulating layer positioned between the two conductive layers. The patent discloses that the intermediate layer may be piezoelectric. It states that where the filament pursuant to the invention is to be provided with piezoelectric characteristics, the core component preferably is compressible, which is achieved either by utilizing hollow filaments or by selection of appropriate synthetic polymers, as for the example, polyolefins with low molecular weight or polyethers. The patent continues to say that a suitable form of execution consists of using as the core component, an electrically conductive, highly viscous liquid with metal and/or carbon black and/or graphite particles dispersed therein. Suitable highly viscous liquids mentioned are, e.g., cis- and transpolyacetylene of relatively low molecular weight.
We have now discovered that an improved piezoelectric coaxial cable results if the core comprises a high molecular weight polymeric material having conductive particles dispersed therein, and the high molecular weight polymer of the core and the piezoelectric polymer are selected such that an intimate bond is formed between the core and the surrounding piezoelectric layer.